The operational amplifier (op amp) is an essential circuit building block of universal importance. Op amps are extremely versatile and are used in innumerable applications. One such application is to amplify an analog input signal to drive a load connected to an output of the op amp.
In a conventional class A operation op amp circuit, selected transistors are employed as bias current providers for providing bias currents. The control terminals of these transistors are coupled to constant bias voltages; hence the bias currents provided by these transistors have fixed values.
As such, after a conventional operational amplifier circuit is properly installed and supply voltages are applied to the operational amplifier circuit, power will be continually consumed by the op amp's bias current components. However, in practice, an op amp circuit may not have a 100% duty cycle. For example, if no signal is input to the operational amplifier, the op amp will not be under operation, and as such no driving operation (current supplied to a load) will be performed by the operational amplifier circuit. Nonetheless, even when the operational amplifier is not under operation, power will be continually consumed by the transistors that are configured to provide the constant bias currents. In other words, power associated with “unused” bias current is wasted.
The foregoing issue is particularly problematic in wireless applications, such as cellular telephones that have limited battery power. When an op amp is used as part of a front end of a receiver in a wireless device, that receiver is preferably capable of handling signals with wide dynamic ranges. With a typical class-A operation op amp, relatively high DC bias current is needed to accommodate received signals. For example, in a given wireless spectrum, a large “blocker” signal may be present adjacent a signal of interest. While such a blocker signal may not always be present, the receiver of the wireless device, i.e., a circuit including an op amp, must nevertheless be capable of handling the larger dynamic range that is driven by the intermittent (or low duty cycle) blocker signal. When the blocker signal is not present, the supplied bias current is wasted, thereby unnecessarily wasting battery power.
It is noted that a class AB operation op amp can provide high output current with low DC bias current, but such a configuration suffers from insufficient linearity, especially in the presence of relatively large blocker signals.
There is accordingly a need to provide an improved class A operation op amp that addresses the undesirable waste of bias current.